284 THE STORY OF THE BIRDS 



birds' eggs, and which we had the pleasure 

 of hearing in Sheffield a quarter of a century 

 ago, is published we are unable to say. 



When the egg-shell is completed, and in the 

 case of spotted or coloured eggs the pigments 

 have been deposited, the egg makes its way 

 from the cloaca out through the vent. It now 

 awaits incubation, or the development of the 

 contained embryo, in a nest or not as the case 

 may be, after which process the perfect chick 

 is duly hatched. The time occupied in this 

 process varies considerably in the different 

 groups. It has been observed that the length 

 of this incubation period depends not only 

 upon the state of perfection in which the 

 chick leaves the egg, but upon the size of 

 the parent (the larger species taking a greater 

 number of days than the smaller ones^) and 

 the prevailing temperature of the area or country 

 in which it takes place. It has been ascer- 

 tained that a reduction of temperature will 

 temporarily arrest the development of the 

 embryo in its earliest stages, whilst a higher 

 temperature than the normal one will not 



^ Size alone is not a determining factor. We must also take 

 into consideration the order or family to which the species belongs 

 as well as the relative size. 



